Southern France is a rather large area. Lots of possibilities. If you mean Provence, you definitely should ride Mount Ventoux.

In the Pyranees, staying in Pau would be a good idea. A number of the major climbs of the TDF are accessible form there, such as the Aubisque, the Tourmelet, etc.

Around the Cote d Azur, the roads along the sea tend to be narrow and very crowded in the summer, so I'd try to head inland from the coast.